In conventional associative stores, the searching operation carried out to determine the position of the recorded words, is carried our by an operation of sequential interrogation; the interrogation time is therefore necessarily lengthy, the more so indeed the larger the capacity of the store.
Optical stores and in particular holographic stores, combine a high storage capacity with the possibility of parallel read-out, in other words simultaneous and therefore fast read-out, of the recorded words.
D. Gabor, in an article entitled "Associative holographic memories," (IBM J. Res. Develop.March 1969), proposed that the principles utilised in holographic correlators in order to achieve the simultaneous identification of the words contained in a store page recorded by holographic method, should be used. It is well known, in other words, that if the interference patterns produced by two coherent light beams A and B which may each contain information in binary form, are recorded, and that if the resulting hologram is read out using a coherent light beam A.sub.1, the beam B will reconstitute the information which it contains, in the form of an auto-correlation signal, with a density which is the higher the closer the information contained in the beam A.sub.1 is to that contained in the beam A.
An associative store system which is not very different from this, has been put forward by SAKAGUCHI et.al. in "A new associative memory system utilising holography" (IEEE Trans. on computers. Vol. C, 19. No. 12, Dec. 1970). The coherent beams A and B, respectively associated with two sets of binary words a.sub.j and b.sub.j, make it possible to record the holograms forming a store page, line by line; a line of order j simultaneously incorporates the information relating to a word a.sub.j and that relating to a word b.sub.j, in the form of as many separate holograms, all comprising the bits of b.sub.j, as there are bits in a.sub.j ; each hologram occupies in the line or row, one or the other of two distinct positions depending upon whether the bit a.sub.j with which it is associated has the value 0 or 1, the bits of a.sub.i thus being coded by position of their associated hologram. At the time of read-out, a coherent beam A.sub.l projects on to the hologram page thus obtained, a set of spots, each spot coinciding in each line or row, with one or the other of the two possible positions of each recorded hologram; all lines of the spots are identical, each containing one and the same key word A.sub.l, coded in the form of its binary complement by the position of the spots in the line. Any hologram coinciding with a spot, will reconstitute the word b.sub.j which has been used to record it. Thus, in a line where the recorded word a.sub.i differs by p of its bits, from the key word a.sub.1, the complement a.sub.l of a.sub.i is projected in the form of a line of spots p of which coincide with the recorded holograms; the line therefore reconstitutes p superimposed images of the word b.sub.i associated with the word a.sub.i. By contrast, if a.sub.j is identical to a.sub.l and therefore differs completely from a.sub. l, no spot in the projected line coincides with a hologram of the line j, and no image of b.sub.j is reconstituted, making it possible, therefore, to identify in the store page the positions of those words a.sub.j which are identical to the key word a.sub.1.
The afore described systems present a double disadvantage. The different words in the holographic storage plane must be recorded successively and this complicates the procedure of recording. Each bit has to be recorded both directly and in the forms of its complement, and this divides the store density by two.